The enigmatic nature of the circumstellar envelope
and bow shock surrounding Betelgeuse as revealed by Herschel. I. Evidence of
clumps, multiple arcs, and a linear bar-like structure

However you pronounce its name*,
the star Betelgeuse is hard to miss on a clear winter's night.
Representing the top left shoulder of Orion the Hunter it blazes a
bright red colour. At over 600 light years away Betelgeuse is not
particularly close, but it shines 100,000 times as brightly as our Sun.

Betelgeuse is a "red supergiant" star which is nearing the end of its
life. As it has swelled in size over the past few hundred thousand
years, currently measuring around 1000 times the size of our Sun, the
massive star has been shedding its outer layers. This material is made
of gas and dust, which has cooled over time and is seen here in
far-infrared light by Herschel.

The ejected outer layers of the star expanded outwards until they hit
the surrounding material, creating the arc-like structures seen to the
left of the image. These arcs are a bow shock, similar to the wave that
travels in front of a ship moving through water, and are caused by
Betelgeuse's motion through the surrounding gas cloud at around 30 km/s
(70,000 mph).

Further to the left is what appears to be a straight wall of gas and
dust, the origin of which is uncertain. It may be material that was shed
by Betelgeuse at an earlier stage - and so has had longer to travel
outwards - but it is then hard to explain why it is so straight. It
appears to be made of gas and dust of the same composition as the arcs
around Betelgeuse, but is slightly cooler, at around -210 Celsius.

The other, more favoured, possibility is that it is just a part of
the cloud that Betelgeuse is moving through, and which is being
illuminated by the star's own starlight. If that is the case then in
around 5000 years the arcs around Betelgeuse will plough into it, and in
around 20,000 years Betelgeuse itself will follow. The gas clouds are
incredibly thin, so such a collision would have no impact Betelgeuse
itself, but the slight increase in density could lead to more stars
being created in the far future.

The immediate surroundings of Betelgeuse look much brighter than the
regions further out, implying that around 30,000 years ago the star
starting shedding mass at a higher rate. The structure in this "inner
envelope" around the star is also assymetric, suggesting that the
material has not been flowing out from the star in a uniform way.

As dramatic as this image looks, Betelgeuse has a far more exciting
future in store. It weighs in at between 10 and 20 times the mass of our
Sun, and such massive stars live fast and die young. Betelgeuse is only
around 10 million years old - a tiny fraction of the Sun's 5 billion
years - but is very much in the twilight of its life. At some point in
the next million years or so (a blink of an eye in astronomical terms!)
the core of the star will run out of fuel, at which point Betelgeuse
will die in one of the most violent events in nature - a supernova.

If anyone is around to see that, it will be a magnificent sight to
behold in the sky. In the meantime, Betelgeuse is providing astronomers
with the opportunity to study an aging, massive star in great detail.

*Some say
"Beetle-juice", others "Bettel-gerz", but it's impossible to know how it
was originally pronounced. The name Betelgeuse literally means "armpit
of the giant" in Arabic, though this is largely due to a historical
mis-translation - its original name meant "shoulder of the great one,
which is somewhat more dignified!

Image:

An annotated version of Herschel's view of Betelgeuse. Image credit: ESA/Herschel/PACS/MESS.